Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!amdcad!ames!amelia!lemming.nas.nasa.gov!fouts
From: fouts@lemming.nas.nasa.gov.nas.nasa.gov (Marty Fouts)
Newsgroups: comp.lang.fortran
Subject: Fortran function side effects (was Re: i++, i+=1, i=i+1)
Message-ID: <1000@amelia.nas.nasa.gov>
Date: 16 Sep 88 16:33:54 GMT
References: <3976@h.cc.purdue.edu> <3659@lanl.gov>
Sender: news@amelia.nas.nasa.gov
Reply-To: fouts@lemming.nas.nasa.gov.nas.nasa.gov (Marty Fouts)
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Keywords: 

In article <3659@lanl.gov> jlg@lanl.gov (Jim Giles) writes:
>
>No, it doesn't cause the subscript to be evaluated twice.  In Fortran
>functions are not allowed to have side effects.

I'm not sure what the standard says about function side effects, but I
just checked a little program and several implementations allowed two
kinds of side effects.  Both modification of variables contained in
common blocks and i/o were allowed by these compilers.  Should I be
reporting this as a compiler bug (and thereby breaking a lot of code
which modifies common data in functions) or does Fortran actually
allow side effects in functions?

Here's the program I tried, which on several machines output

 I =     1
 I =     1 J =     1

and on no machine generated a compiler error, or output

 I =     1 J =     0

Marty

      PROGRAM SIDETEST
C
C Can functions have side effects?
C
      COMMON /SIDE/ K
      K = 0
      I = J(1)
      WRITE(6,100) I, K
 100  FORMAT(' I = ', I5, ' J = ', I5)
      END
C
      FUNCTION J(I)
      COMMON /SIDE/ K
      WRITE (6,100) I
 100  FORMAT(' I = ', I5)
      J = I
      K = I
      RETURN
      END

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